How equals method returns false when comparing two objects: e1.equals(e2)

Question

class Emp { 
    public Emp(String s) {

    }
}

public class Testing { 
    public static void main(String[] args) { 
        Emp e1 = new Emp("hello"); 
        Emp e2 = new Emp("hello"); 
        System.out.println(e1 == e2); 
        System.out.println(e1.equals(e2));

        String s1 = new String("hello");
        String s2 = new String("hello");
        System.out.println(s1 == s2);
        System.out.println(s1.equals(s2));
    }

}

OUTPUT IS: false false false true

Answer 1

In order for e1.equals(e2) to return true, either the default implementation of Object's equals should return true (which requires that e1==e2), or you should override equals in the Emp class in a way that would return true when applied to your e1 and e2 instances.

Answer 2

If you don't override the methods hashcode and equals, the default equals method will return true if they are located at the same memory location. As e1 and e2 are separate objects, they are at separate memory locations and thus without an override equals, e1.equals(e2) will be false.

Answer 3

The "Emp" class inherits the equals() method from the root "OBJECT" class.

Below is the equals() method code present in Object class :

public boolean equals(Object obj)
{
    return (this == obj);
}

As you can see above which checks for the memory location address for equality.

First case :

Emp e1 = new Emp("hello"); 
Emp e2 = new Emp("hello"); 
System.out.println(e1 == e2); 

Here you are comparing the reference( memory location ), since you have created two new objects in heap, there memroy location are different and hence it retuned "false"

Second case :

Emp e1 = new Emp("hello"); 
Emp e2 = new Emp("hello"); 
System.out.println(e1.equals(e2));

Here you are involing the inherited equals() method from the "Object" class which inturn check for the references ( memory location ) of these two objects and hence retuned false.

Third case :

    String s1 = new String("hello");
    String s2 = new String("hello");
    System.out.println(s1 == s2);

In this case the String class has the equals() method overridden which actually check for the object fields for comparison, below is the source code for equals() method in String class :

public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) {
        String anotherString = (String)anObject;
        int n = count;
        if (n == anotherString.count) {
            char v1[] = value;
           char v2[] = anotherString.value;
            int i = offset;
            int j = anotherString.offset;
            while (n-- != 0) {
                if (v1[i++] != v2[j++])
                    return false;
            }
            return true;
        }
    }
    return false;
}

This is similar to first case, here you are comparing the reference( memory location ) of two string objects, since you have created two new objects in heap, there memroy location are different and hence it retuned "false"

Fourth case :

    String s1 = new String("hello");
    String s2 = new String("hello");
    System.out.println(s1.equals(s2));

Here you are invoking the overridden equals() method from the string class ( the code pasted above ). if you see the logic it first actually checks if the two refrences are referring to same memory location if same it returns true, else it will actually check the contents of the two string objects which is backed by the charcter array, it compares each and every character with the coressponding charcter of these tow string objects, since both the objects have same contents " hello", it returned true.

As Ken mentioned above please go through the equals() and hashcode() contract.

Answer 4

        System.out.println(e1 == e2);  <-- A
        System.out.println(e1.equals(e2)); <-- B
        System.out.println(s1 == s2); <-- C
        System.out.println(s1.equals(s2)); <-- D

1. Expression A => returning false. This is pretty obvious, because this expression is comparing two distinct object
2. Expression B => returning false. Every class implicitly extends Object class. Therefore, the implementation of the equals method is using the one defined in Object class which is the same with the expression A
3. Expression C => returning false. The reason is the same with the Expression A comparing two distinct object
4. Expression D => returning true. Because String class has its own implementation of equals function which check the value of the string.